The present invention relates generally to a detecting system for detecting fuel gas flow rate.
In the automotive technology, evaporative emission control systems have been known in the art. Such systems are designed for preventing fuel gas generated in fuel tanks from being discharged to atmosphere. Namely, fuel gas generated in the fuel tank is introduced into a canister so as to be absorbed by activated carbon therein. The fuel evaporative emission is then introduced into an air induction passage of an engine through a fuel gas purge passage for combustion in the engine. Furthermore, in Japanese Unexamined Patent Publication (Kokai) 2-136558, there is disclosed a self-diagnosis system in the evaporative emission control system. The self-diagnosis system operates a purge valve disposed in the evaporative emission purge passage when the pressure in the fuel tank is higher than or equal to a predetermined pressure, to make judgement whether abnormality is present or not based on variation of an air/fuel ratio. Namely, in the self-diagnosis system, judgement whether abnormality, such as blocking of the purge passage and so forth, is present or not, after assuring generation of the fuel evaporative emission. Therefore, it is possible to employ a fuel gas flow rate detecting device in place of a pressure sensor for detecting the internal pressure of the fuel tank. As a flow rate sensor for substantially small flow rate, it may be possible to measure substantially small flow rate of fuel gas employing a strain gauge as proposed in Japanese Unexamined Patent Publication No. 63-236922 and Japanese Unexamined utility Model Publication No. 1-180620, for example.
However, in the system employing such a strain gauge to detect the flow rate from its small deformation magnitude, it is required to prepare a strain gauge which was manufactured with a substantially high accuracy, for obtaining a satisfactorily high sensitivity. Therefore, this method lacks practical applicability. Reference may be made to copending U.S. Patent application entitled "Self-diagnosis system in evaporated fuel gas distribution preventing system" filed on basis of Japanese patent application No. 3-75413 (of the filing date Apr. 8, 1991); and copending U.S. patent application entitled "FUEL EVAPORATIVE EMISSION AMOUNT DETECTION SYSTEM" filed on basis of Japanese patent application No. 3-79763 (of the filing date Apr. 12, 1991), respectively filed in behalf of Nippon Denso Co., Ltd. (the assignee of the present application).